The present invention relates to a current feedback type electronic impedance circuit.
An example of a known current feedback type electronic impedance circuit is shown in the official gazette of Japanese Laid-Open Patent Application No. 52-116052 (corresponding to U.S. Pat. No. 4,220,875). This known circuit comprises a voltage-current converter which is constructed of a differential amplifier, and a variable gain current amplifier which detects the variations of the outputs of the voltage-current converter and which feeds its output signal back to the voltage-current converter. The input impedance of the electronic impedance circuit as viewed from the input terminal thereof can be varied by controlling the gain of the variable gain current amplifier.
The inventors have studied the above-described prior-art electronic impedance circuit and have determined that a drawback exists with it. Specifically, when the control current of the variable gain current amplifier for controlling the input impedance has a very small value on the order of several .mu.A, the base current of the transistor of the voltage-current converter is no longer negligible relative to the minute control current. As a result, a d.c. offset arises in the input d.c. voltage of the voltage-current converter. When the signal of such an electronic impedance circuit in this case is amplified by a d.c. amplifier, a very great d.c. offset arises in the output of the d.c. amplifier.